Current target designs approaches use isolated lines and isolated lines with assist features for sensitivity enhancement. Software generates a library of simulated spectra, using material properties (n and k of resist, antireflective coatings, planarization films, etc.), nominal test target structure (line width, sidewall angle and height), reasonable expected range of target structure variation, and scatterometry tool optical parameters (wavelength range, azimuth angle range, angle of incidence range, polarizations, etc.). FEM (focus exposure matrix) test wafers, produced on this basis, are then exposed and spectra are collected from test targets for all combinations (if possible) of focus and exposure in the FEM. Closest match for each spectrum is found in the library. Corresponding structure parameters (line width, sidewall angle, line height, etc.) for the library matches are matched up with the programmed focus and exposure combinations from the FEM. The resulting target sensitivity is not sufficient to provide the required accuracy and precision even for the recent node. The sensitivity enhancement of the printed pattern to the scanner focus changes achieved due to using of assist features as described e.g., in T. A. Brunner, C. P. Ausschnitt, Process Monitor Gratings. Proc. of SPIE Vol. 6518, 2007 and in U.S. Pat. No. 7,916,284 may be limited by a printability of the pattern in a whole range of scanner parameters and allowable range of process parameters.
Line end shortening effect measured by imaging tool (Christopher P. Ausschnitt, Mark E. Lagus, Seeing the forest for the trees: a new approach to CD control. SPIE Vol. 3332, 1998) suffers from a low sensitivity especially around the best scanner focus position where the behavior of measured target parameter is parabolic as a function of the scanner focus change.
Use of phase shift masks (see T. A. Brunner, et al., “Quantitative stepper metrology using the focus monitor test mask”, Proc. of SPIE, Vol. 2197, 1994 and U.S. Pat. No. 7,545,520 as examples) allows achieving a very high sensitivity to scanner focus changes by converting of focus changes into alignment errors that can be measured with an overlay measurement tool. However phase shift masks are not used in production since it leads to a significant rise in reticle prices.
Current target designs require some trade-off between target sensitivity and target printability which ends up with a relatively low sensitivity of printed patterns to the scanner focus changes, thus current methods have either low precision or low accuracy or long “time to result”.
U.S. Patent Application Publication No. 2014/0141536, which is incorporated herein by reference in its entirety, discloses a segmented mask that includes a set of cell structures, wherein each cell structure includes a set of features having an unresolvable segmentation pitch along a first direction, wherein the unresolvable segmentation pitch along the first direction is smaller than the illumination of the lithography printing tool, wherein the plurality of cell structures have a pitch along a second direction perpendicular to the first direction, and wherein the unresolvable segmentation pitch is suitable for generating a printed pattern for shifting the best focus position of the lithography tool by a selected amount to achieve a selected level of focus sensitivity.